Top administrators at the Environmental Protection Agency have reversed the stance of their agency's experts on the safety of a chemical that may be used to fumigate fruit in California in the fight against the Mediterranean fruit fly.

Last December, EPA proposed to ban most uses of the chemical, ethylene dibromide (EDB), because it causes cancer and birth defects in animals. The EPA staff estimated then that about two persons in 10,000 across the nation would probably get cancer because of the chemical's use on different crops.

The additional risk to the country if all California citrus were fumigated was put at five cancer cases per 100,000 population, in a document recently prepared by EPA scientists. Several thousand cancer cases might be expected to result eventually, an EPA staff member said.

However, a top pesticide administrator, who asked not to be identified, said the agency now takes the position that the chemical is safe for widespread use without any special precautions. He said he felt staff estimates of the risk were 50 times too high and that a more accurate estimate would be closer to one cancer case per million or less. He said that risk, considered very low on the scale of regulated substances, was acceptable when compared with the economic losses California could face if its crops were not fumigated.

Fumigation has already begun in California, mostly using a chemical closely related to ethylene dibromide, but somewhat less toxic and far more expensive to use. Ethylene dibromide is used chiefly for citrus fruits, plums, and cherries -- crops grown extensively in other parts of California's Central Valley, but only in small amounts in the five counties quarantined so far because of the Medfly infestation. The use of ethylene dibromide is expected to increase if spraying extends to the rest of the Central Valley.

The pesticide administrator said he also disagreed with staff scientists over whether special precautions should be taken if there is a massive fumigation program in California. The staff said that at least six days should elapse between the time the sprayed fruit is shipped and when it reaches the market -- a move that would keep the fruit off the market for an extra four or five days. The chemical gradually evaporates from within the fruit over that time. If it weren't held up, the EPA staff said its risk estimate would have to be revised upward.

The administrator, however, said that normal shipping times would be sufficient to help reduce the amount of chemical in the fruit, and no extra time is necessary.

Joseph Panetta, project manager for work on the chemical, stood by the staff work, saying, "We think our estimate is as accurate as possible based on the evidence."

The pesticide administrator who commented on the subject said that all such estimates are uncertain, and that "we had some concern about these staff figures." Since the figures used in the proposed ban were for lifetime consumption of fruit and grain treated with the chemical, and the fumigation in California would be only a one-year project, the estimates would be hard to recalculate downward from low lifetime exposures to higher one-year exposures.

EPA estimates that failure to use the chemical for fumigation in California could result in a $250 million loss, from crops damaged and from fumigation with other, more expensive chemicals.

Fruit and vegetables from a quarantined area, such as the five counties in California, may be put on the market only after being fumigated or being given a cold-temperature treatment to assure that no fruit fly larvae are still alive inside the fruit. If they were alive, they might quickly infest the area where the crops were sold. It appears that the Medfly outbreak in Florida was started in just this way -- infested fruit was shipped from California to Florida, where the pest hatched.

EPA's proposal to ban ethylene dibromide must go through a public comment period before the agency will make a decision.